System for selectively coupling an energy source to a load and method of making same

ABSTRACT

A multi-energy storage device system includes an electric drive coupled to a load, a DC link coupled to the electric drive, and a bi-directional voltage converter having an output channel coupled to the DC link and an input channel. A first energy storage device (ESD) is coupled to the input channel, and a switch is coupled to the DC link and to a second ESD. A system controller causes the switch to couple the second ESD to the DC link for delivering energy stored in the second ESD to the electric drive. The system controller also causes the voltage converter to convert a voltage of the first ESD to a higher voltage and to deliver the higher voltage to the DC link, wherein the higher voltage is greater than the voltage of the second ESD and causes the switch to decouple the second ESD from the DC link.

BACKGROUND

Embodiments of the invention relate generally to drive systems and, more specifically, to selectively coupling an energy source to a load to supply energy thereto in a vehicle or non-vehicle system.

Electric vehicles and hybrid electric vehicles are typically powered by one or more energy storage devices, either alone or in combination with an internal combustion engine. In pure electric vehicles, the one or more energy storage devices powers the entire drive system, thereby eliminating the need for an internal combustion engine. Hybrid electric vehicles, on the other hand, include energy storage device power to supplement power supplied by an internal combustion engine, which greatly increases the fuel efficiency of the internal combustion engine and of the vehicle. Traditionally, the energy storage devices in electric or hybrid electric propulsion systems include batteries, ultracapacitors, flywheels, or a combination of these elements in order to provide sufficient energy to power an electric motor.

When two or more energy sources are used to provide power to drive system, the energy sources are typically well-suited to provide different types of power. A first energy source, for example, may be a high specific energy source that is more efficient or economical at providing long-term power while a second energy source may be a high specific-power source more efficient at providing short-term power. The high specific-power source may be used to assist the high specific energy source in providing power to the system during, for example, acceleration or pulsed load events.

Often, the high specific-power source is directly coupled to the direct current (DC) link that supplies a voltage to a load. However, control of the DC link voltage is dependent on the directly coupled high power source and can be lower than a desired response. In addition, the transient power required from the high energy source to be supplied to the DC link can be higher than a desired response.

Therefore, it is desirable to provide a system that allows selective coupling of an energy source to a load to supply energy thereto.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one aspect of the invention, a multi-energy storage device system includes an electric drive coupled to a load, a DC link coupled to the electric drive, and a bi-directional voltage converter having an output channel coupled to the DC link and an input channel. A first energy storage device is coupled to the input channel of the voltage converter, a switch is coupled to the DC link, and a second energy storage device is coupled to the switch. A system controller is configured to cause the switch to couple the second energy storage device to the DC link such that energy stored in the second energy storage device is delivered to the electric drive and such that a voltage of the second energy storage device is reduced as the energy stored therein is delivered to the electric drive. The system controller also causes the bi-directional voltage converter to convert a voltage of the first energy storage device to a higher voltage and to deliver the higher voltage to the DC link, wherein the higher voltage is greater than the voltage of the second energy storage device and causes the switch to decouple the second energy storage device from the DC link.

In accordance with another aspect of the invention, a method of assembling a propulsion energy system includes coupling an output channel of a bi-directional voltage converter assembly to a direct current (DC) link and coupling a first energy storage device to a first input channel of the bi-directional voltage converter assembly. The bi-directional voltage converter assembly includes a first bi-directional voltage converter. The method also includes coupling a switch to the DC link, coupling second energy storage device to the switch, and coupling a load to the DC link. The load is configured to receive energy from one of the first energy storage device and the second energy storage device via the DC link. The method further includes coupling a controller to the bi-directional voltage converter assembly and to the switch and configuring the controller to cause the switch to couple the second energy storage device to the DC link such that energy stored in the second energy storage device is delivered to the load and such that a voltage of the second energy storage device is reduced as the energy stored therein is delivered to the load. The controller is also configured to cause the bi-directional voltage converter to convert a voltage of the first energy storage device to a higher voltage and to deliver the higher voltage to the DC link, wherein the higher voltage is greater than the voltage of the second energy storage device and causes the switch to decouple the second energy storage device from the DC link.

In accordance with another aspect of the invention, a non-transitory computer readable storage medium having a computer program stored thereon and representing a set of instructions that when executed by a computer causes the computer to activate a first switch to transfer a voltage stored in a second energy storage device to a direct current (DC) link coupled to a load such that the voltage stored in the second energy storage device is delivered to the load as a voltage of the second energy storage device is reduced. The instructions also cause the computer to control a bi-directional voltage converter to convert a first voltage of a first energy storage device to a second voltage and to deliver the second voltage to the DC link, wherein the second voltage is greater than the voltage of the second energy storage device and causes the first switch to deactivate.

Various other features and advantages will be made apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate preferred embodiments presently contemplated for carrying out the invention.

In the drawings:

FIG. 1 schematically illustrates an embodiment of a propulsion system according to an embodiment of the invention.

FIG. 2 schematically illustrates another embodiment of a propulsion system according to an embodiment of the invention.

FIG. 3 schematically illustrates another embodiment of a propulsion system according to an embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention relate to vehicle and non-vehicle applications. Vehicular applications may include pure-electric or hybrid-electric vehicle applications in, for example, on-road and off-road vehicles, golf cars, neighborhood electric vehicles, forklifts, and utility trucks as examples. Non-vehicular applications may include non-vehicular types of loads including pumps, fans, winches, cranes, or other motor driven loads. While described with respect to the vehicular applications, embodiments of invention are not intended to be limited to such.

FIG. 1 illustrates a propulsion system 100 according to an embodiment of the invention. Propulsion system 100 may be used in electric or hybrid vehicle applications. Vehicle propulsion system 100 includes an energy system 102 and a system controller 104. Energy system 102 includes a first energy storage device 106, a second energy storage device 108, and a bi-directional DC-DC voltage converter 110 having an input channel 112 coupled to first energy storage device 106 and having an output channel 114 coupled to a DC link 116. First energy storage device 106 may be used to provide longer-lasting energy while second energy storage device 108 may be used to provide higher-power energy for acceleration, for example. While first energy storage device 106 is illustrated as a battery, another type of energy storage devices such as an ultracapacitor, a fuel cell, a flywheel, or the like is also contemplated. While second energy storage device 108 is illustrated as an ultracapacitor, another type of energy storage devices such as a battery, a fuel cell, a flywheel, or the like is also contemplated.

First energy storage device 106 is coupled via DC link 116 to a load 118, which, according to an embodiment of the invention, is an electric drive including a DC-AC inverter 120 and a motor or electromechanical device 122. Motor 122 is preferably an AC motor but is not limited as such. While not shown, it is to be understood that each of a plurality of motors 122 may be coupled to a respective wheel or other load or that each motor 122 may be coupled to a differential for distributing rotational power to the wheels or other load.

Generally, in a motoring mode of operation, voltage converter 110 also acts to boost the voltage provided by a low voltage side 124 of energy system 102 to a high voltage side 126 of energy system 102. That is, voltage from first energy storage device 106 is provided to voltage converter 110 via a bus 128 coupled to input channel 112 thereof on the low voltage side 124 of energy system 102. The provided voltage is boosted by voltage converter 110 such that the voltage provided to DC link 116 on the high voltage side 126 of energy system 102 is increased to an operating level of electric drive 118. Electric drive 118 inverts the voltage on DC link 116 and provides the inverted voltage to electromechanical device 122.

In an accelerating mode of operation, energy from high-power second energy storage device 108 is generally desired to be used instead of or in addition to the voltage provided by first energy storage device 106 via voltage converter 110. Accordingly, propulsion system 100 includes a switch 130 configured to selectively couple and decouple second energy storage device 108 to/from DC link 116. Coupling second energy storage device 108 to DC link 116 allows second energy storage device 108 to discharge its stored energy to take advantage of the higher power available therefrom during acceleration. In one embodiment, second energy storage device 108 is an ultracapacitor in which its charge is affected by its voltage. Second energy storage device 108 preferably includes a plurality of series- and parallel-connected capacitor cells in this embodiment where each capacitor cell has a capacitance greater than 100 Farads per cell. Because the charge of second energy storage device 108 is affected by its voltage, as the voltage in the second energy storage device 108 is supplied to DC link 116, the charge of second energy storage device 108 reduces, thus reducing the voltage thereof. As the voltage of second energy storage device 108 reduces, the corresponding voltage on DC link 116 also reduces accordingly. While second energy storage device 108 is coupled to DC link 116 via switch 130, voltage converter 110 is not free to establish the voltage on DC link 116 without simultaneously charging second energy storage device 108. However, by decoupling second energy storage device 108 from DC link 116, voltage converter 110 is free to establish the voltage on DC link 116 without charging second energy storage device 108, which allows faster control of the voltage on DC link 116 and lowers the transient power supplied from first energy storage device 106.

Accordingly, during the accelerating mode operation, system controller 104 is programmed to close switch 130 to couple second energy storage device 108 to DC link 116. In one embodiment, system controller 104 is programmed to cause the voltage on DC link 116 to substantially match the state of charge or voltage of second energy storage device 108 prior to closing switch 130 such as, for example, by boosting the voltage from first energy storage device 106. In the illustrated embodiment, switch 130 includes a pair of silicon-controlled rectifiers (SCRs) 132, 134 coupled together and arranged in an anti-parallel arrangement. While a pair of SCRs is shown, it is contemplated that other switching devices as known in the art may be used. System controller 104 is thus programmed to activate SCR 132 (such as by applying a gate voltage thereto) such that voltage from second energy storage device 108 may be supplied to load 118 via DC link 116. Once activated, SCR 132 tends to remain in an “on” or closed state while current flowing therethrough remains above a holding current thereof.

When the acceleration is finished or when a state of charge or voltage of second energy storage device 108 falls to or below a threshold, for example, system controller 104 is programmed to cause switch 130 to change to an “off” or deactivated state to decouple second energy storage device 108 from DC link 116. To turn off SCR 132, the current flowing therethrough should be lowered to below its holding current. To accomplish this, system controller 104 is programmed to cause voltage converter 110 to boost the voltage of first energy storage device 106 to a voltage higher than the voltage of second energy storage device 108. Increasing the voltage on DC link 116 to a voltage higher than the voltage of second energy storage device 108 in this manner causes the current flowing through SCR 132 to fall below its holding current. As such, SCR 132 is caused to turn off, thus decoupling second energy storage device 108 from DC link 116.

In a decelerating mode of operation in which the speed of rotation of motor 122 is to be decreased to zero or to a lower speed from its current speed, system controller 104 is programmed to operate electric drive 118 in a regenerative mode (such as by operating electromechanical device 122 in a generator mode), wherein electric power or energy is returned to DC link 116 through DC-AC inverter 120 during a regenerative braking event. According to embodiments of the invention, system controller 104 causes the regenerative braking energy to be delivered to second energy storage device 108 via switch 130 to increase its state of charge or voltage. That is, system controller 104 is programmed to activate SCR 134 (such as by applying a gate voltage thereto) such that voltage from load 118 may be supplied to second energy storage device 108 via DC link 116. Similar to SCR 132, once activated, SCR 134 tends to remain in an “on” or closed state while current flowing therethrough remains above a holding current thereof.

When the deceleration is finished or when a state of charge or voltage of second energy storage device 108 rises to or above a threshold, for example, system controller 104 is programmed to cause switch 130 to change to an “off” or deactivated state to decouple second energy storage device 108 from DC link 116. To turn off SCR 134, the current flowing therethrough should be lowered to below its holding current. To accomplish this, in one embodiment, propulsion system 100 includes a dynamic retarder 136 coupled to DC link 116 that has a low-resistance resistor 138 and switch 140. System controller 104 is programmed to close or modulate switch 140 such that the current flowing through SCR 134 falls below its holding current. As such, SCR 134 turns off, thus decoupling second energy storage device 108 from DC link 116. System controller 104 may then open switch 140 to prevent current from continuing to flow through dynamic retarder 136.

In another embodiment, system controller 104 may cause voltage converter 110 to draw current from DC link 116 such that the current flowing through SCR 134 falls below its holding current. As such, SCR 134 is cause to turn off, thus decoupling second energy storage device 108 from DC link 116.

In another embodiment, DC-AC inverter 120 controls reduce the DC-AC inverter's DC output voltage to below that of energy storage device 108, thus causing SCR 134 to turn off and thus decoupling energy storage device 108 from DC link 116.

Closed loop power control may be used to determine the power split between the energy sources 106, 108, and (if present) dynamic retarder 136.

FIG. 2 illustrates a propulsion system 142 according to another embodiment of the invention. Elements and components common to traction systems 100 and 142 will be discussed relative to the same reference numbers as appropriate.

In addition to components 102-140 common with propulsion system 100, propulsion system 142 includes a charge circuit 144 coupled to DC link 116 and coupled to second energy storage device 108. If second energy storage device 108 is decoupled from DC link 116, system controller 104 is programmed to cause charge circuit 144 to charge second energy storage device 108 from the voltage on DC link 116 such that the full state of charge or voltage of second energy storage device 108 may be reached. System controller 104 may optimize the charging of second energy storage device 108 such that the effect on first energy storage device 106 is minimized. Additionally, charge circuit 144 may be used during the regenerative braking event to charge second energy storage device 108 using a more controlled operation than merely causing SCR 134 to conduct and when there is a desire to maintain a higher DC link voltage than the present capacitor voltage. A preferred embodiment of charge circuit 144 would be a buck/boost converter allowing controlled charging of energy storage device 108 from a voltage below that of the DC link to a voltage above the DC link.

FIG. 3 illustrates a propulsion system 146 according to another embodiment of the invention. Elements and components common to traction systems 100, 142, and 146 will be discussed relative to the same reference numbers as appropriate.

FIG. 3 shows that charge circuit 144 may be coupled directly to first energy storage device 106 rather than to DC link 116 as shown in FIG. 2. In this embodiment, the losses of the boost converter may be avoided during charging of storage device 108. A preferred embodiment utilizes boost or buck boost chargers to allow the voltage of storage device 108 to be charged from a voltage above or below that of storage device 106 to a voltage above that of storage device 106.

One skilled in the art will appreciate that system controller 104 may be implemented via a plurality of components such as one or more of electronic components, hardware components, and/or computer software components. These components may include one or more tangible computer readable storage media that generally stores instructions such as software, firmware and/or assembly language for performing one or more portions of one or more implementations or embodiments. Examples of a tangible computer readable storage medium include a recordable data storage medium and/or mass storage device. Such tangible computer readable storage medium may employ, for example, one or more of a magnetic, electrical, optical, biological, and/or atomic data storage medium. Further, such media may take the form of, for example, floppy disks, magnetic tapes, CD-ROMs, DVD-ROMs, hard disk drives, and/or electronic memory. Other forms of tangible computer readable storage media not listed may be employed with embodiments of the invention.

A number of such components can be combined or divided in an implementation of the systems described herein. Further, such components may include a set and/or series of computer instructions written in or implemented with any of a number of programming languages, as will be appreciated by those skilled in the art.

A technical contribution for the disclosed method and apparatus provides for a computer-implemented device capable of selectively coupling an energy source to a load to supply energy thereto in a vehicle or non-vehicle system.

Therefore, according to an embodiment of the invention, a multi-energy storage device system includes an electric drive coupled to a load, a DC link coupled to the electric drive, and a bi-directional voltage converter having an output channel coupled to the DC link and an input channel. A first energy storage device is coupled to the input channel of the voltage converter, a switch is coupled to the DC link, and a second energy storage device is coupled to the switch. A system controller is configured to cause the switch to couple the second energy storage device to the DC link such that energy stored in the second energy storage device is delivered to the electric drive and such that a voltage of the second energy storage device is reduced as the energy stored therein is delivered to the electric drive. The system controller also causes the bi-directional voltage converter to convert a voltage of the first energy storage device to a higher voltage and to deliver the higher voltage to the DC link, wherein the higher voltage is greater than the voltage of the second energy storage device and causes the switch to decouple the second energy storage device from the DC link.

According to another embodiment of the invention, a method of assembling a propulsion energy system includes coupling an output channel of a bi-directional voltage converter assembly to a direct current (DC) link and coupling a first energy storage device to a first input channel of the bi-directional voltage converter assembly. The bi-directional voltage converter assembly includes a first bi-directional voltage converter. The method also includes coupling a switch to the DC link, coupling second energy storage device to the switch, and coupling a load to the DC link. The load is configured to receive energy from one of the first energy storage device and the second energy storage device via the DC link. The method further includes coupling a controller to the bi-directional voltage converter assembly and to the switch and configuring the controller to cause the switch to couple the second energy storage device to the DC link such that energy stored in the second energy storage device is delivered to the load and such that a voltage of the second energy storage device is reduced as the energy stored therein is delivered to the load. The controller is also configured to cause the bi-directional voltage converter to convert a voltage of the first energy storage device to a higher voltage and to deliver the higher voltage to the DC link, wherein the higher voltage is greater than the voltage of the second energy storage device and causes the switch to decouple the second energy storage device from the DC link.

According to yet another embodiment of the invention, a non-transitory computer readable storage medium having a computer program stored thereon and representing a set of instructions that when executed by a computer causes the computer to activate a first switch to transfer a voltage stored in a second energy storage device to a direct current (DC) link coupled to a load such that the voltage stored in the second energy storage device is delivered to the load as a voltage of the second energy storage device is reduced. The instructions also cause the computer to control a bi-directional voltage converter to convert a first voltage of a first energy storage device to a second voltage and to deliver the second voltage to the DC link, wherein the second voltage is greater than the voltage of the second energy storage device and causes the first switch to deactivate.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

What is claimed is:
 1. A multi-energy storage device system comprising: an electric drive coupled to a load; a direct current (DC) link coupled to the electric drive; a bi-directional voltage converter comprising an output channel coupled to the DC link and comprising an input channel; a first energy storage device coupled to the input channel of the bi-directional voltage converter; a switch coupled to the DC link; a second energy storage device coupled to the switch; and a system controller configured to: cause the switch to couple the second energy storage device to the DC link such that energy stored in the second energy storage device is delivered to the electric drive and such that a voltage of the second energy storage device is reduced as the energy stored therein is delivered to the electric drive; and cause the bi-directional voltage converter to convert a voltage of the first energy storage device to a higher voltage and to deliver the higher voltage to the DC link, wherein the higher voltage is greater than the voltage of the second energy storage device and causes the switch to decouple the second energy storage device from the DC link.
 2. The system of claim 1 wherein the system controller is further configured to: cause the switch to couple the second energy storage device to the DC link such that energy generated in the electric drive is delivered to the second energy storage device and such that a voltage of the second energy storage device is increased as the energy is delivered thereto from the electric drive; and cause the bi-directional voltage converter to lower a current flowing through the switch to cause the switch to decouple the second energy storage device from the DC link.
 3. The system of claim 1 wherein the system controller is further configured to: cause the switch to couple the second energy storage device to the DC link such that energy generated in the electric drive is delivered to the second energy storage device and such that a voltage of the second energy storage device is increased as the energy is delivered thereto from the electric drive; and cause the inverter to lower a voltage on the DC link to a voltage lower than a voltage of the second energy storage device to cause the switch to decouple the second energy storage device from the DC link.
 4. The system of claim 1 further comprising a dynamic retarder coupled to the DC link; and wherein the system controller is further configured to: cause the switch to couple the second energy storage device to the DC link such that energy generated in the electric drive is delivered to the second energy storage device and such that a voltage of the second energy storage device is increased as the energy is delivered thereto from the electric drive; and cause the dynamic retarder to lower a current flowing through the switch to cause the switch to decouple the second energy storage device from the DC link.
 5. The system of claim 4 wherein the dynamic retarder comprises: a resistor; and a switching device.
 6. The system of claim 1 wherein the switch comprises a pair of silicon controlled rectifiers coupled together and arranged in an anti-parallel configuration.
 7. The system of claim 1 further comprising a charge circuit coupled to the second energy storage device; and wherein the system controller is further configured to cause the charge circuit to increase the voltage of the second energy storage device if the second energy storage device is decoupled from the DC link.
 8. The system of claim 7 wherein the charge circuit is further coupled to the DC link and configured to convert a voltage of the DC link to a charging voltage and to supply the charging voltage to the second energy storage device to increase the voltage thereof.
 9. The system of claim 7 wherein the charge circuit is further coupled to the first energy storage system and configured to convert a voltage of the first energy storage system to a charging voltage and to supply the charging voltage to the second energy storage device to increase the voltage thereof.
 10. The system of claim 1 wherein the system controller is further configured to cause the bi-directional voltage converter to boost a voltage from the first energy storage device and to supply the boosted voltage to the DC link prior to causing the switch to couple the second energy storage device to the DC link, wherein the boosted voltage is substantially equal to a voltage of the second energy storage device.
 11. The system of claim 1 wherein the first energy storage device comprises a battery; and wherein the second energy storage device comprises an ultracapacitor comprising a plurality of series- and parallel-connected capacitor cells, each capacitor cell having a capacitance greater than 100 Farads.
 12. A method of assembling a propulsion energy system, the method comprising: coupling an output channel of a bi-directional voltage converter assembly to a direct current (DC) link, the bi-directional voltage converter assembly comprising a first bi-directional voltage converter; coupling a first energy storage device to a first input channel of the bi-directional voltage converter assembly; coupling a switch to the DC link; coupling second energy storage device to the switch; coupling a load to the DC link, the load configured to receive energy from one of the first energy storage device and the second energy storage device via the DC link; coupling a controller to the bi-directional voltage converter assembly and to the switch; and configuring the controller to: cause the switch to couple the second energy storage device to the DC link such that energy stored in the second energy storage device is delivered to the load and such that a voltage of the second energy storage device is reduced as the energy stored therein is delivered to the load; and cause the bi-directional voltage converter to convert a voltage of the first energy storage device to a higher voltage and to deliver the higher voltage to the DC link, wherein the higher voltage is greater than the voltage of the second energy storage device and causes the switch to decouple the second energy storage device from the DC link.
 13. The method of claim 12 further comprising configuring the controller to set a voltage of the DC link via boost control of the voltage of the first energy storage device.
 14. The method of claim 12 wherein the first energy storage device comprises a battery; and wherein the second energy storage device comprises an ultracapacitor.
 15. The method of claim 14 wherein the ultracapacitor is configured to provide a higher power than the battery.
 16. The method of claim 12 further comprising: coupling a dynamic retarder to the DC link; and configuring the controller to: cause the load to operate in a regenerative braking mode and to supply a regenerative braking voltage to the DC link; cause the switch to couple the second energy storage device to the DC link such that the regenerative braking voltage is delivered to the second energy storage device and such that a voltage of the second energy storage device is increased as the regenerative braking voltage is delivered thereto; and cause the dynamic retarder to lower a current flowing through the switch to cause the switch to decouple the second energy storage device from the DC link.
 17. The method of claim 12 wherein coupling the switch to the DC link comprises coupling a pair of silicon controlled rectifiers arranged in an anti-parallel configuration to the DC link.
 18. The method of claim 12 further comprising coupling a charge circuit the second energy storage device; and configuring the controller to cause the charge circuit to increase the voltage of the second energy storage device if the second energy storage device is decoupled from the DC link.
 19. A non-transitory computer readable storage medium having a computer program stored thereon and representing a set of instructions that when executed by a computer causes the computer to: activate a first switch to transfer a voltage stored in a second energy storage device to a direct current (DC) link coupled to a load such that the voltage stored in the second energy storage device is delivered to the load as a voltage of the second energy storage device is reduced; and control a bi-directional voltage converter to convert a first voltage of a first energy storage device to a second voltage and to deliver the second voltage to the DC link, wherein the second voltage is greater than the voltage of the second energy storage device and causes the first switch to deactivate.
 20. The computer readable storage medium of claim 19 wherein the set of instructions further causes the computer to: activate the first switch to transfer a voltage on the DC link generated by the load to the second energy storage device such that the voltage of the second energy storage device is increased; and activate a dynamic retarder switch in a dynamic retarder coupled to the DC link to reduce a current flowing through the switch to a level less than a holding current of the first to deactivate the first switch.
 21. The computer readable storage medium of claim 20 wherein the set of instructions further causes the computer to activate a charge circuit coupled to the second energy storage device when the second energy storage device is decoupled from the DC link to cause the charge circuit to increase the voltage of the second energy storage device. 